"We understand that this incident may be the result of a global cyber fraud operation, and we are cooperating closely with the United States Secret Service and Department of Justice," says Robert H.B. Baldwin Jr., Heartland's president and CFO. Baldwin said about 100 million card transactions per month occur on the affected systems which provide processing to merchants and businesses.

Heartland has set up a Web site to provide information about the data breach and advise cardholders to examine their monthly statements and report suspicious activity to their card issuers. Heartland does not believe that its payroll or micropayments operations were impacted by the data breach.

Baldwin says the computer forensics conducted by the company has uncovered evidence of multiple instances of malicious software on the Heartland network, although he didn't disclose the exact number of identified instances.

Baldwin apologized for the inconvenience caused by the situation in which cardholder numbers, and sometimes names, were stolen.

The company is taking steps to improve its network security by adding what it referred to as "a next-generation program designed to flag network anomalies in real-time" to better identify possible criminal activity but didn't go into details.